The present invention relates to a motor vehicle power supply device for energizing a starter motor when an engine mounted on a motor vehicle is to be started.
Lead storage batteries are mounted on motor vehicles as a power supply for an engine starter and other electric loads such as accessories.
When the engine is to be started, electric energy stored in the lead storage battery is supplied to energize the starter motor. A pinion gear of the starter motor is brought into mesh with a ring gear mounted on the crankshaft of the engine, and rotated to rotate the crankshaft, thereby starting the engine.
An electric current which is supplied from the battery to the starter motor when starting the engine is very high, e.g., 100A or more, though it is supplied in a short period of time. The capacity of a battery to be installed on a motor vehicle is determined primarily in view of its ability to start the engine. The large electric power which is consumed to start the engine is supplemented when the battery is charged by electric power generated by an alternator mounted on the motor vehicle and driven by the engine while the motor vehicle is running.
Batteries mounted on motor vehicles are known lead batteries as secondary batteries, and they are charged and discharged through a chemical reaction between electrodes and an electrolytic solution. Such a battery can discharge a large current within a short period of time. The battery is charged with a current of 10A or less which is supplied over a long period of time and through a gradual chemical reaction. Therefore, if a much larger current is supplied to charge the battery, the battery would be excessively heated and the electrodes might be deformed and damaged.
Motor vehicles which are mainly used by commuters run over short distances, and motor vehicles used as delivery cars are repeatedly stopped and started highly frequently. Since these motor vehicles require the engines to be started frequently and are continuously driven over short periods of time, the batteries mounted on these motor vehicles cannot be charged sufficiently enough to make up for the electric power consumed when the engines are started. Accordingly, the batteries tend to be used up, or run down, failing to start the engines.
To solve the above problems, the applicant has proposed a motor vehicle power supply device which has a large-capacitance capacitor that is charged by a battery mounted on the motor vehicle and that discharges stored electric energy to actuate the engine starter to start the engine (see Japanese Patent Application No. 63(1988)-329,846, U.S. patent application Ser. No. 454,267 and EPC Patent Application No. 89313559.0.
With the proposed motor vehicle power supply devices, the engine starter is energized by the electric energy stored in the large-capacitance capacitor. The large-capacitance capacitor can supply an intensive current. Therefore, even if the voltage of the battery is somewhat lowered, the engine may be started by the electric energy stored in the capacitor. To charge the capacitor after its stored electric energy has been discharged, a sudden large current is required since the internal resistance of the capacitor is small at the time it is charged, and hence the alternator is subjected to a large stress. While the engine is idling at low temperature, the engine may possibly die if the load on the alternator is large. If the alternator load is large, the engine may also not be revved up with a quick response when the motor vehicle is to be started or accelerated.